In a power conversion device, in many cases, from the viewpoint of circuit protection, a high-voltage block and a low-voltage block are electrically separated from each other by using a communication medium such as a transformer or an optical module. Particularly, a configuration using an optical module is often used for industry since it is not easily influenced by noise. The operation rate of an industrial power conversion device is high and the causes of failures of the power conversion device are mainly an insulation failure and temperature abnormality in an inverter. In a power conversion circuit, in most of the cases, the failures are degradation failures by switching erroneous operation and accumulation of temperature abnormality of a switching element due to the switching erroneous operation. Patent literature 1 discloses an abnormality detecting method of generating a gate monitor signal (feedback signal) on the basis of gate voltage and gate current at the time of switching operation and diagnosing whether switching elements operate normally or not by a signal processing unit. In the method, when abnormality is detected, the operation of all of switching elements is forcedly stopped to protect the switching elements.
In the conventional art, however, a technique of determining whether switching elements operate normally from a gate monitor signal and calculating also the temperature of the switching elements without using a new signal path is not disclosed.